The present invention generally relates to waveguide filters of the type using plural dual mode resonant cavities, and more particularly to an improvement in such filters which makes it possible to realize the general class of coupled cavity transfer functions.
U.S. Pat. No. 3,697,898 to Blachier and Champeau describes a dual mode circular and/or square waveguide filter having input and output ports located at the two physical ends of the filter. More specifically, the Blachier and Champeau filter uses N physical waveguide cavities which resonate in two independent orthogonal modes and are coupled together to provide the filtering capacity of n=2N electrical cavities resonating in a single mode. Intra cavity coupling is provided by a structural discontinuity such as a screw mounted in the cavity wall. Inter cavity coupling is provided by means of selective polarization discriminating couplings between the N cavities to transfer energy between identical modes in the coupled cavities. A particular feature of the Blachier and Champeau filter is the use of a phase inversion means in coupled cavities to provide a subtraction capability between identical modes in the coupled cavities. This substration capability can provide steep response skirts for the passband of the filter.
The Blachier and Champeau filter has significant advantages not only in an improved passband response but also in economies in weight and volume and in the ease of fabrication which results from the realization of two electrical cavities in one physical cavity. However, the Blachier and Champeau filter structure will not realize the general class of coupled cavity transfer functions since no provision to couple electrical cavities 1 and n, 2 and n-1 and so forth are provided. For a development of a synthesis procedure for the general class of canonical waveguide filters, the reader is referred to the article by Atia, Williams and Newcomb entitled "Narrow-Band Multiple-Coupled Cavity Synthesis" published in the IEEE Transactions on Circuits and Systems, Vol. CAS-21, No. 5, September 1974, at pages 649 to 655. The Blachier and Champeau filter is not capable of generating the optimum response and, therefore, the full potential of this particular waveguide cavity structure is not realized.